Lean-burn gasoline engines can be more efficient and thus use less fuel and produce less carbon dioxide than corresponding engines operating under stoichiometric conditions.
To approach to treat engine emissions is to catalytically convert NO to a solid, prototypically barium nitrate, and store it in an emission control device during lean operation. The device is regenerated periodically by briefly shifting engine operation to stoichiometric or rich conditions, under which the barium nitrate becomes released NO that is then reduced. The operating temperature range for the device can be determined by the activity of the catalyst used to form the solid nitrate (defining the lower limit) and the stability of the nitrate under lean conditions (defining the upper limit). A typical range is approximately 200 to 500° C.
Although the device works well initially, its performance typically degrades over time. One reason for this is a slow accumulation of sulfate, derived from the combustion of fuel sulfur, which effectively competes with the nitrate for storage space. The sulfate is more stable than the nitrate, but it can be removed by an occasional exposure to rich conditions at a somewhat higher temperature than that used for normal regeneration of the trap.
The inventors herein, however, have recognized another reason for the degradation in performance of the device. Specifically, there can be a loss in activity of the catalyst used to form the solid nitrate. For example, if the catalyst is platinum supported on a high-surface-area-oxide, its loss in activity can result from loss of platinum surface area due to coarsening of the supported particles of platinum.
As such, the inventors herein have recognized a disadvantage with prior approaches for removing sulfur contamination. Specifically, the oscillation of exhaust air-fuel ratio at high exhaust temperatures can result in exposure of the emission control device to conditions which increase particle growth. As one example, the inventors herein have recognized that lean exposure at high temperatures above a lean limit value (that varies as a function of temperature) can result in such particle growth.
The above disadvantages with prior approaches are overcome by a method for controlling an engine coupled to an emission control device susceptible to sulfur contamination, the method comprising:
deciding whether to reduce sulfur contamination in the device based on at least an operating condition;
in response to a decision to reduce sulfur contamination:
raising temperature of the device by adjusting engine operation; and
when said temperature reaches a preselected value, oscillating an air-fuel ratio entering the device between rich and lean to reduce said sulfur contamination, where an amplitude of said air-fuel oscillations is determined based on exhaust temperature.
In this example approach, it is possible to limit the amplitude of the air-fuel oscillations to reduce exposing the device to conditions that increase particle growth, while at the same time still allowing desulfurization.
Note that this limitation of air-fuel ratio amplitude can be one sided, in that only the lean amplitude is limited. Alternative, to maintain symmetry, both the lean and rich peaks can be limited.